Method of forming composite solder by cold compaction and composite solder

ABSTRACT

A reinforced solder is formed by mixing particles of a solder material capable of forming a metal matrix and a reinforcing particulate to form a particulate mixture; compressing the mixture at room temperature to form a solid compact; and sintering the compact to form a particulate composite in which the reinforcing particulate is embedded in a metal matrix formed from the solder material. The solder material can be a lead-free solder material. There is also provided a reinforced solder which includes a lead-free metal matrix and a reinforcing particulate embedded in the metal matrix. The metal matrix includes a combination of tin, copper, silver, and indium having volume ratios of about 91.4:0.5:4.1:4. The reinforcing particulate has an average diameter less than 100 nm. The reinforced solder has a melting point between 180° C. to 230° C.

FIELD OF THE INVENTION

The present invention relates generally to solders, and particularly tocomposite solders and methods of forming composite solders.

BACKGROUND OF THE INVENTION

Composite solders include both a solder material and a reinforcementmaterial. The solder material is typically an eutectic orlow-melting-temperature alloy and forms a metallic matrix. Thereinforcement material is typically dispersed in the solder matrix inthe form of particulates. Composite solders have improved propertiesover monolithic solders, which typically have some deficiencies due, forexample, to their coarse microstructural features.

Conventional processes for synthesizing composite solders includemelting a solder material, adding a reinforcement material to the soldermaterial, and stirring the resulting mixture, to facilitate the uniformdispersion of the reinforcement material in the mixture. The mixture isthen solidified to a predetermined shape. In some cases, a wetting agentis added to the mixture. A drawback of such processes is that theresulting solder has inferior physical and mechanical properties thancan be realized using powder based methods. Moreover, it is difficult toincorporate and uniformly disperse particulates of reinforcementmaterial that are smaller than about one micron. It is also difficult toproduce composite solder materials with high reproducibility andreliability. The production cost can be high. Further, contamination orundesirable chemical reactions can occur during melting or when thewetting agent is added.

Accordingly, there is a need for an improved method of synthesizingcomposite solders and for composite solders having improved properties.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provideda method of forming reinforced solder. The method includes mixingparticles of a solder material capable of forming a metal matrix and areinforcing particulate to form a particulate mixture. The mixture iscompressed at room temperature to form a solid compact, which issintered to form a particulate composite in which the reinforcingparticulate is embedded in a metal matrix formed from the soldermaterial. The solder material can be a lead-free solder material.

In another aspect of the invention, there is provided a reinforcedsolder formed in accordance with this method.

In accordance with a further aspect of the invention, there is provideda reinforced solder including a lead-free metal matrix and a reinforcingparticulate embedded in the metal matrix. The metal matrix includes acombination of tin, copper, silver, and indium having volume ratios ofabout 91.4:0.5:4.1:4. The reinforcing particulate has an averagediameter less than 100 nm. The reinforced solder has a melting pointbetween 180° C. to 230° C.

Other aspects and features of the present invention will become apparentto those of ordinary skill in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate exemplary embodiments of the invention,

FIG. 1 is a plurality of field emission scanning electron microscopy(FESEM) images of exemplary composite solders.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

An exemplary embodiment of the invention is a method of forming areinforced composite solder, wherein a mixture of particles of a soldermaterial and a reinforcing particulate is cold compacted to form acompact. The compact is a particulate composite having the reinforcingparticulate embedded in a metal matrix formed by the solder material.

As used herein, a particulate composite refers to a composite materialcomposed of particles embedded in a matrix. A metal matrix refers to amass of metal particles in which a reinforcing particulate can beembedded. The metal matrix can hold the compact together in a stableshape.

The solder material can be primarily made of an alloy suitable forsoldering and capable of forming a metal matrix when compressed andsubsequently sintered. The solder material melts at relatively lowtemperatures, such as below 220° C. Any such alloy known to personsskilled in the art may be used. For example, lead-free alloys, such astin-based alloys, may be suitable. A tin-based alloy may include one ormore other metals such as antimony, bismuth, copper, indium, silver,zinc, and the like. As is known, solder alloy having an eutecticcomposition is advantageous because a low melting temperature is usuallydesirable. An example of suitable alloys is a Viromet™ 349 alloy, whichhas volume ratios Sn/Cu/Ag/In=91.4/0.5/4.1/4. As is known, when thevolume ratio of the alloy is larger than 0.5, it will likely form ametal matrix on compaction.

A particulate herein refers to a substance or matter in the form ofseparate and fine particles. The reinforcing particulate has smallparticle sizes. The term “particle size” as used herein refers to theaverage diameter of the particles. As the particles may havenon-spherical shapes and different sizes, the term “diameter” refers tothe average or effective diameter. An effective diameter of anon-spherical particle is the diameter of a spherical particle that hasthe same volume as the non-spherical diameter. Small particle sizestypically refer to sizes less than 100 nm. However, depending on theapplication and the desired properties of the end product, particlessizes of the reinforcement particulate can be in the range of submicronsor microns.

The reinforcing particulate is made of one or more materials that can beembedded in the metal matrix formed from the solder particles such thatthe resulting particulate composite has a melting temperature suitablefor soldering and has one or more improved properties over a monolithicsolder formed from the solder alloy only. For example, the reinforcingmaterial should be stable, and should not transform into a differentphase, at the soldering temperatures of the composite solder. Exampleproperties that can be improved by addition of the reinforcingparticulate include density, porosity, mechanical strength (0.2 yieldstrength (YS) and ultimate tensile strength (UTS)), dimensional andthermal stability, ductility, and the like. As is known, the reinforcingparticulate may also include any material that, when embedded in thesolder matrix, can suppress grain-boundary sliding, improveintermetallic compound morphology, reduce grain growth, and redistributeinternal stress more uniformly. The reinforcement material may have amelting temperature higher than the melting temperature of the solderalloy so that during soldering the reinforcement material can remain insolid form.

Many materials that can be used to reinforce composite solders are knownto persons skilled in the art. Materials that have been used in otherexisting composite solders as reinforcement materials may be used.Example suitable reinforcement materials include ceramic materials suchas alumina (aluminium oxide, Al₂O₃), titanium dioxide (TiO₂), and thelike. Some metals, alloys and intermetallics may also be suitable.However, reactive materials, unstable materials or materials having lowmelting temperatures are generally not suitable.

The choice of reinforcement material may depend on the particularapplication for which the resulting composite solder is to be used. Ascan be understood, within a limit, a higher proportion of thereinforcing particulate in the mixture may produce a higherreinforcement effect; however, the proportion of the reinforcingparticulate in the mixture should not be too high because when theproportion of the reinforcing particulate is too high, the properties ofthe resulting composite solder can be adversely affected. Further, ifthe content of reinforcing material is too high, it may be difficult orimpossible to form metal matrix from the solder material. Experimentsshow that including up to 5% of reinforcing particulate may producecomposite solders having improved properties. However, in differentembodiments, the proportions of the reinforcing particulate may vary,depending on the materials used, the size of the reinforcingparticulate, the type of application and the desired properties of theend product.

Particles of the solder material, which are capable of forming a metalmatrix, and the reinforcing particulate can be mixed to form aparticulate mixture. The mixture may be blended so as to uniformlydisperse the particles of the solder material and the reinforcingparticulate. Particulates of the solder material and the reinforcementmaterial, and their mixture, can be obtained or formed using techniquesknown to persons skilled in the art, such as those described in thefollowing articles: D. C. Lin et al., Journal of Metastable andNanocrystalline Materials, (2005), vol. 23, pp. 145-148; Japanese patentno. 6031486 to K. Sasapi and K. K. K. D. Tanaka, published Feb. 8, 1994;and U.S. Pat. No. 5520752 to Jr. G. K. Lcey et al., published May 28,1996, each of which is incorporated herein by reference.

The mixture is compressed at a temperature beneath the melting point ofthe solder material. Typically, the mixture is compressed at roomtemperature. Compressing at room temperature can be less expensive andeasy to perform. The mixture can be compressed using a technique knownas cold compaction, which typically includes applying a pressure to themixture, for example, with a hydraulic press. Cold compaction generallyis more particularly described in J. P. Schaffer et al., The Science andDesign of Engineering Materials, WCB/McGraw-Hill, 1999, pp. 689-691,which is incorporated herein by reference. Typically, the pressure isapplied steadily. However, it may be applied cyclically. Cold compactionunder cyclic pressure can be advantageous and can lead to improvedcompact density, a reduction in the density gradient and a more uniformreinforcement distribution. The compact may be of any desirable shapeand size. It can be in the form of an ingot. Optionally, the compact maybe coated with colloidal graphite to minimize oxidation.

The compact may then be sintered in an inert environment, such as in aninert gas. As can be understood, sintering can cause particles to bindand thus make the compact denser. The compact can be sintered atsuitable temperatures which may vary depend on the particularcircumstances but should not be too high to avoid melting the compactmelts or otherwise significantly deteriorating the physical propertiesof the resulting composite solder. For example, the sinteringtemperature can be in the range of about 70% to 90% of the absolutemelting temperature of the compact. Sintering in an inert environmentcan prevent or reduce oxidation of the compact.

The compacts may be extruded into a desired shape such as a cylindricalsolder rod. The extrusion temperature, which can vary between roomtemperature and about 160° C. depending on the desired property of theend product, and the extrusion ratio can be varied depending on thedesirable rod sizes.

As can be appreciated, the forming process does not require melting ofthe solder particulate. Nor is it necessary to add wetting agents.Therefore, some of the problems associated with the conventionalprocesses for forming composite solders can be avoided. For example,contamination or undesirable chemical reactions can be reduced oravoided. Production cost can also be reduced.

The resulting composite solders have a metal matrix and reinforcingparticulate embedded therein and exhibit improved properties over amonolithic solder consisting of the solder alloy only. To illustrate,the results of tests performed on sample composite solders formed in anexemplary process are shown in FIG. 1 and Tables 1 to 3.

The sample composite solders were formed as follows.

Mixtures of particulate of Viromet 349 alloy and particulate of aluminahaving particle sizes of about 50 nm were blended at a typical speed of50 rpm for about 10 hours. The proportion of the alumina particulatevaried from one (1) to five (5) percent by volume.

The blended mixtures were cold compacted into ingots, typically of 35 mmdiameter and 40 mm length, by applying a typical pressure of 50 tonsusing a 150-ton hydraulic press.

The compacted ingots were coated with colloidal graphite and weresubsequently sintered at a temperature of about 150° C. for about two(2) hours in an argon gas.

The sintered ingots were extruded into cylindrical rods, some at roomtemperatures and others at about 150° C.

The resulting sample solders were tested and examined for theirmicrostructural and physical properties including mechanical properties.Some results of these tests are shown in Tables 1 to 3 and FIG. 1, incomparison with results obtained from a pure VIROMET solder sample. Thesamples I to IV were extruded at room temperature and the samples I′ toIV′ were extruded at about 150° C.

FIG. 1 shows FESEM images of samples II to IV. As can be seen andappreciated by persons skilled in the art from images in the leftcolumn, the alumina particulates (shown as small white dots) wererelatively uniformly distributed in the composite solder samples. As canbe appreciated from images in the right column, near-equiaxedintermetallic phases with good integrity are present in the metallicmatrix. Images in both columns show that the metallic matrix in eachsample has pores in the submicron and nano range rather than micron sizepores. Further, as shown in the Tables, in comparison to Viromet 349,the density (hence the weight) of the composite solder samples werereduced (up to ˜7.5%; for 5 volume % alumina); their CTEs were reduced(up to ˜16%; for 5 volume % alumina), hence enhancing the dimensionalstability; their hardness was increased (up to ˜18.5%; for 5 volume %alumina); and their strength was significantly increased. The 0.2% yieldstrength was increased by up to ˜31% for 5 volume % alumina, and theultimate tensile strength was increased by up to ˜28% for 3-5 volume %alumina).

Other features, benefits and advantages of the present invention notexpressly mentioned above can be understood from this description andthe drawings by those skilled in the art.

Of course, the above described embodiments are intended to beillustrative only and in no way limiting. The described embodiments aresusceptible to many modifications of form, arrangement of parts, detailsand order of operation. The invention, rather, is intended to encompassall such modification within its scope, as defined by the claims. TABLE1 Structural Properties Mixture Al₂O₃ Density Porosity Pore Size SampleMaterials Wt % (g/cc) (%) (μm) I Pure 0.0 7.202 ± 0.042 1.14 0.26VIROMET II VIROMET/ 0.53 7.182 ± 0.075 3.3 0.56 1% Al₂O₃ III VIROMET/1.63 6.948 ± 0.074 2.1 0.33 3% Al₂O₃ IV VIROMET/ 2.74 6.665 ± 0.035 3.030.85 5% Al₂O₃ I′ VIROMET 0.0 6.773 ± 0.369 4.50 0.73 II′ VIROMET/ 0.536.621 ± 0.193 1.66 0.58 1% Al₂O₃ III′ VIROMET/ 1.63 6.598 ± 0.159 2.170.33 3% Al₂O₃ IV′ VIROMET/ 2.74 6.329 ± 0.129 6.31 0.52 5% Al₂O₃

TABLE 2 Coefficient of Thermal Expansion (CTE), Hardness and SecondaryPhases Microhardness Sample CTE (Hv) Phases Present I 33.261 ± 1.64 15.8± 0.526 Sn, Ag₃Sn, Ag II 30.639 ± 2.79 16.8 ± 1.080 Sn, Ag₃Sn, Ag III30.443 ± 0.31 16.9 ± 0.801 Sn, Ag₃Sn, Ag IV 27.832 ± 2.54 18.7 ± 0.482Sn, Ag₃Sn, Ag I′ 29.790 ± 1.68 16.6 ± 0.785 Sn, Ag₃Sn, Ag II′ 27.629 ±3.22 15.2 ± 1.525 Sn, Ag₃Sn, Ag III′ 27.250 ± 0.11 15.4 ± 0.150 Sn,Ag₃Sn, Ag IV′ 26.962 ± 0.42 15.5 ± 0.051 Sn, Ag₃Sn, Ag

TABLE 3 Tensile Properties Sample (%) 0.2% YS (MPa) UTS (MPa) DuctilityI 56 ± 6 60 ± 8 37 ± 7 II 72 ± 6 75 ± 6 21 ± 3 III 73 ± 3 77 ± 3 11 ± 3IV 74 ± 3 76 ± 2 10 ± 0

1. A method of forming reinforced solder, comprising: mixing particlesof a solder material capable of forming a metal matrix and a reinforcingparticulate to form a particulate mixture; compressing said mixture atroom temperature to form a solid compact; and sintering said compact toform a particulate composite in which said reinforcing particulate isembedded in a metal matrix formed from said solder material.
 2. Themethod of claim 1, wherein said solder material is lead free soldermaterial.
 3. The method of claim 2, wherein said solder material isViromet.
 4. The method of claim 1, wherein said reinforcing particulatecomprises a ceramic.
 5. The method of claim 4, wherein said ceramiccomprises Al₂O₃.
 6. The method of claim 5, wherein said reinforcingparticulate has an average diameter less than 100 nm.
 7. The method ofclaim 1, further comprising extruding said compact.
 8. The method ofclaim 1, wherein said mixing comprises blending said mixture.
 9. Themethod of claim 1, wherein said compact has a melting temperature at afirst temperature and said sintering comprises heating said compact inan inert gas at second temperature between about 70% to about 90% ofsaid first temperature.
 10. The method of claim 9 wherein said secondtemperature is between about 84° C. and 163° C.
 11. The method of claim9, wherein said second temperature is about 150° C.
 12. The method ofclaim 2, wherein said lead free solder material is a lead-free metalalloy.
 13. The method of claim 12, wherein said lead-free metal alloycomprises an combination of tin, copper, silver, and indium.
 14. Themethod of claim 5, wherein the proportion of said reinforcingparticulate is between 0 to about 5% by volume of said mixture.
 15. Themethod of claim 1, further comprising coating said compact withcolloidal graphite.
 16. The method of claim 1, wherein said reinforcingparticulate is made of a material having a density lower than thedensity of said solder material.
 17. A reinforced solder formed inaccordance with the method of claim
 1. 18. A reinforced soldercomprising a lead-free metal matrix and a reinforcing particulateembedded in said metal matrix, said metal matrix comprises a combinationof tin, copper, silver, and indium having volume ratios of about91.4:0.5:4.1:4, said reinforcing particulate having an average diameterless than 100 nm, and said reinforced solder having a melting pointbetween 180° C. to 230° C.
 19. The reinforced solder of claim 18,wherein said reinforcing particulate is made of alumina and has anaverage diameter of about 50 nm.
 20. The reinforced solder of claim 19,wherein said reinforced solder has a density lower than that of amonolithic solder formed from a material having said combination. 21.The reinforced solder of claim 19, wherein said reinforced solder hasporosity up to 6.3%.
 22. The reinforced solder of claim 19, wherein saidreinforced solder has a coefficient of thermal expansion lower than thatof a monolithic solder formed from a material having said combination.23. The reinforced solder of claim 19, wherein said reinforced solderhas hardness higher than that of a monolithic solder formed from amaterial having said combination.
 24. The reinforced solder of claim 19,wherein said reinforced solder has 0.2% yield strength higher than 56MPa.
 25. The reinforced solder of claim 19, wherein said reinforcedsolder has an ultimate tensile strength higher than 60 MPa.
 26. Thereinforced solder of claim 19, wherein said reinforced solder hasductility lower than 37%.